System and method for calibrating an optical extensometer

Abstract

An exemplary video extensometer is disclosed that includes a load string configured to attach a test specimen, an imaging device configured to capture images of a surface of the test specimen when attached to the load string, a storage device configured to store a plurality of first calibration parameters corresponding to intrinsic characteristics of the imaging device, and a control circuit configured to perform a verification process using the first calibration parameters to verify that the plurality of second calibration parameters correspond to an arrangement of the test specimen relative to the imaging device, and to perform an optical strain measurement process to measure a displacement of the test specimen based on the first calibration parameters and the second calibration parameters.

Description

[Technical field]

[0001] Field of Disclosure FIELD OF THE DISCLOSURE This disclosure relates generally to optical extensometers, and more particularly to systems and methods for calibrating optical extensometers. [Background technology]

[0002] background Camera-based vision systems have been implemented as part of materials testing systems for measuring specimen elongation and / or strain. These systems collect one or more images of the specimen under test. These images are synchronized with other signals of interest for the test (e.g., specimen load, machine actuator / crosshead displacement, etc.). The images of the test specimen can be analyzed to identify and track the location of specific features of the specimen as the test progresses. Changes in the location of such features, such as changes in the relative position of one or more reference features of the specimen, allow the local specimen deformation to be calculated, which in turn allows the specimen strain to be calculated.

[0003] Conventional systems employ cameras or other imaging systems to capture images from which to measure properties of a test sample. However, imaging and / or measurement differences between a known reference position and an actual position can lead to distorted readings and inaccurate measurements. Thus, a system that corrects for such errors is desired. Summary of the Invention

[0004] overview A system and method for calibrating an optical extensometer is disclosed substantially as shown in and described in connection with at least one of the figures.

[0005] BRIEF DESCRIPTION OF THE DRAWINGS The benefits and advantages of the present invention will become readily apparent to those of ordinary skill in the art after considering the following detailed description and accompanying drawings. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a block diagram of an exemplary extensometer system according to aspects of the present disclosure.

[0007] [Diagram 2] FIG. 2 illustrates an exemplary test sample measured in the extensometer system of FIG. 1 according to an embodiment of the present disclosure.

[0008] [Diagram 3] FIG. 2 is a block diagram of another view of the example extensometer system of FIG. 1 according to an embodiment of the present disclosure.

[0009] [Figure 4] FIG. 2 is a block diagram of an exemplary implementation of the extensometer system of FIG. 1 according to an aspect of the present disclosure.

[0010] [Figure 5A] FIG. 2 illustrates an exemplary arrangement of the imaging device and calibration plate of FIG. 1 that can be used to calibrate an optical extensometer system for intrinsic properties and store the resulting calibration parameters.

[0011] [Figure 5B] FIG. 2 illustrates an exemplary arrangement of the imaging device and calibration plate of FIG. 1 that can be used to calibrate an optical extensometer system for intrinsic and extrinsic properties at multiple predetermined working distances and store the resulting calibration parameters.

[0012] [Figure 6A] FIG. 5B illustrates an exemplary arrangement of the imaging device and validation plate of FIGS. 1, 5A, and / or 5B that can be used to validate and adjust calibration parameters associated with a configured working distance.

[0013] [Figure 6B]FIG. 5C shows another example arrangement of the imaging device of FIG. 5B and a load string with verification markers that can be used to verify and adjust calibration parameters associated with a configured one of the predetermined working distances.

[0014] [Figure 6C] FIG. 6B illustrates an example arrangement of the imaging device of FIG. 6A and a load string with verification markers that can be used to verify and adjust calibration parameters associated with a configured one of the predetermined working distances.

[0015] [Figure 7] 2 is a plan view of an example base for a kinematic mounting system for a lens assembly of the example imaging device of FIG. 1.

[0016] [Figure 8] 8 is a plan view of an example lens mount for a kinematic mounting system for a lens assembly of the example imaging device of FIG. 1 configured to connect to the base of FIG. 7 in a precise and reproducible manner.

[0017] [Figure 9] FIG. 9 is a perspective view of the example lens mount of FIG. 8.

[0018] [Figure 10] 10 is a cross-sectional view of the lens mount and base of the example imaging assembly of FIGS. 7-9, showing the nesting of the lens mount's bearings with seats in the base. FIG.

[0019] [Figures 11A-11C] 8A-8C show an exemplary progression of reproducible installation of the lens mount of FIG. 8 on the base of FIG. 7.

[0020] [Figure 12A]2 illustrates an example of the imaging device of FIG. 1 including an adjustable imaging device mount mounted inside the housing for adjusting the position and / or rotation of the imaging device's optical sensor and lens assembly.

[0021] [Figure 12B] 10 illustrates another example of the imaging device of FIG. 1 including an adjustable imaging device mount attached to the exterior of the housing for adjusting the position and / or rotation of the imaging device's optical sensor and lens assembly.

[0022] [Figure 13] FIG. 2 is a flow diagram depicting an example method that may be performed to determine and store intrinsic calibration parameters for the extensometer system of FIG. 1.

[0023] [Figure 14] FIG. 2 is a flow diagram depicting an example method that may be performed to determine an extrinsic second calibration parameter for the extensometer system of FIG. 1 based on the calibration parameters.

[0024] [Figure 15] FIG. 2 is a flow diagram depicting an example method that may be performed to determine and store intrinsic and extrinsic calibration parameters for the extensometer system of FIG. 1.

[0025] [Figure 16] FIG. 2 is a flow diagram depicting an example method that may be performed to verify the configuration of the installed optical extensometer system of FIG. 1 based on stored intrinsic and extrinsic calibration parameters. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The figures are not necessarily to scale. Where appropriate, similar or identical reference numbers are used to refer to similar or identical components.

[0027] Detailed Description The present disclosure describes systems and methods for compensating for errors in video extensometer systems, including noise, perspective variation, and / or component placement and / or movement.

[0028] Conventional systems are subject to one or more errors when testing and measuring one or more physical properties of a test sample. The errors may be due to limitations of the system components (e.g., physical / operational limitations of the components, operational effects on related components, etc.), system calibration (e.g., for measuring different materials / samples), and / or measurement and / or analytical limitations (e.g., collection and analysis of the measured properties, etc.).

[0029] Some conventional testing systems employ camera-based vision systems to capture information (e.g., measurements of one or more properties or geometric variables) during a material testing process (e.g., to determine strain of a test specimen). Such systems may capture multiple images of the test specimen and synchronize these images with other information related to the testing process (e.g., specimen load, machine actuator / crosshead displacement, etc.). The images of the test specimen may then be analyzed via one or more algorithms to identify and / or locate specific features of the test specimen (including reference features) and even track such features as the testing operation progresses. Changes in the absolute and / or relative locations of such features allow local specimen deformation to be calculated, which in turn allows specimen strain to be computed.

[0030] The sample features of interest may consist of markings (e.g., fiducial features) applied to the surface of the test sample that are visible to the camera(s). For example, the processor may analyze the image to determine the location and / or shape of the markings (and their changes) and track the movement of these marks relative to one another during testing. For example, multiple markings may be present on the front surface of the sample, such as pair groupings for the determination of gage-length based strain measurements (axial marks, lateral marks, etc.), or pseudo-random speckle patterns used in digital image correlation (DIC) techniques. An alternative set of features that may be of interest for the determination of lateral sample strain are the edges of the test sample.

[0031] For single or multiple camera measurement systems, a calibration process can be performed on a selected calibration plane located at a predetermined distance from the image sensor. The calibration process establishes a relationship between one or more characteristics (e.g., size, position, width, etc.) captured by the imaging device and one or more physical characteristics (e.g., determined in physical coordinates) on the calibration plane.

[0032] A conventional calibration process may employ a calibration reference device placed on a calibration plane. The reference device includes predetermined physical characteristics with known geometric dimensions associated with covering some or all of a field of view (FOV) of interest. The calibration process allows an image of the calibration device to be captured and compared to the known calibration device geometry, where a transfer function is established to transform the coordinates of the image from a pixel coordinate system to a real-world physical coordinate system.

[0033] Conventional video extensometer systems track and measure the dimensions and / or relative locations of markings on the surface of a test specimen. During the testing process, image processing algorithms are executed (e.g., via a processor in the video extensometer system) to determine the locations of the markings on the surface of the specimen. Based on the determined locations, the processor can calculate the initial sample gage length and the instantaneous change in the sample gage length (e.g., axial and / or lateral strain) from the value(s) at the start of the test specimen. The precision with which the video extensometer system can measure the absolute and / or relative positions and / or position changes of the markings depends, at least in part, on whether the surface of the specimen is coplanar with the plane being calibrated. The difference between the location of the measurement plane (corresponding to the surface of the test specimen) and the location of the calibration plane (corresponding to the reference plane) will result in measurement errors (e.g., perspective errors). Larger deviations between the measurement plane and the reference plane (e.g., along the Z-axis between the specimen and the camera) will result in larger measurement errors.

[0034] Conventional calibration techniques can be burdensome for operators of video extensometer systems, at least in part, due to the types of changes to the system that can result in recalibration being required to maintain accuracy. Although some conventional systems, when configured for calibration, allow the calibration process to be performed relatively quickly and easily, the process of installing the required calibration hardware can also be burdensome.

[0035] The disclosed exemplary video extensometer reduces the burden of calibration and recalibration on the operator by performing at least a portion of the calibration prior to installation. To maintain the validity of this calibration, the disclosed systems and methods provide a lens that is mounted on the video extensometer in a manner that provides consistency in position and orientation regardless of differences in operator skill or ability. The disclosed exemplary video extensometer further reduces the burden of calibration by reducing the types of events that can trigger a recalibration, and by reducing the burden of the recalibration itself by using smaller, simpler calibration standards.

[0036] An exemplary optical extensometer disclosed includes a load string configured to attach a test specimen, an imaging device configured to capture an image of a surface of the test specimen when attached in the load string, a storage device configured to store a plurality of first calibration parameters corresponding to intrinsic characteristics of the imaging device, and a control circuit configured to perform a verification process using the first calibration parameters to verify that the plurality of second calibration parameters correspond to an arrangement of the test specimen relative to the imaging device, and to perform an optical strain measurement process to measure a displacement of the test specimen based on the first calibration parameters and the second calibration parameters.

[0037] In some exemplary optical extensometers, the imaging device includes a housing, an image sensor within the housing, and a lens configured to mount to the housing using a kinematic mount. In some exemplary optical extensometers, the kinematic mount is configured to replicate a position and orientation of the lens corresponding to a calibration procedure associated with generating the first calibration parameters. In some exemplary optical extensometers, the kinematic mount includes a base including a plurality of seats and a lens mount including a plurality of bearings having a complementary arrangement with the plurality of seats. In some exemplary optical extensometers, the base and the lens mount further include complementary keying to prevent mismatch of the plurality of bearings with the plurality of seats. In some exemplary optical extensometers, the base includes a plurality of first magnets and the lens mount includes a plurality of second magnets configured to interact with the first magnets to mate the plurality of bearings with the plurality of seats.

[0038] Some exemplary optical extensometers further include an adjustable imaging device mount configured to allow adjustment of a working distance between the imaging device and the load string. In some exemplary optical extensometers, the imaging device includes a housing, an optical sensor within the housing, and a lens mount having a fixed distance relative to the optical sensor, and the adjustable imaging device mount is mounted externally to the housing. In some exemplary optical extensometers, the imaging device includes a housing, an optical sensor within the housing, and a lens mount having a fixed distance relative to the optical sensor, and the adjustable imaging device mount is mounted internally to the housing and configured to adjust the position of the optical sensor and the lens mount. In some exemplary optical extensometers, the control circuitry is configured to perform a verification process and output a signal indicative of whether the working distance is within one or more predetermined working distance threshold ranges.

[0039] In some example optical extensometers, the control circuitry is configured to select one of a plurality of sets of second calibration parameters stored in the storage device corresponding to a plurality of predetermined working distances based on performing the validation process. In some example optical extensometers, the control circuitry is configured to calculate the second calibration parameters based on the first calibration parameters and using a predetermined validation specimen attached to the load string.

[0040] In some example optical extensometers, the intrinsic characteristic of the imaging device includes one or more of a focal length, an optical center, a distortion parameter, a pixel size, or a pixel skew parameter. In some example optical extensometers, the second calibration parameter includes a relative position of the imaging device and the load string with respect to a reference position and a relative rotation of the imaging device and the load string with respect to a reference orientation. In some example optical extensometers, the control circuitry is configured to perform the verification process based on a predetermined calibration plate placed in the load string. In some example optical extensometers, the control circuitry is configured to perform the verification process based on a fiducial marker located on the load string. Some example optical extensometers further include an adjustable imaging device mount configured to allow adjustment of the focal length by adjusting a distance between a lens of the imaging device and the optical sensor, the adjustable imaging device mount including a plurality of discrete adjustment points.

[0041] Another disclosed example optical extensometer includes a load string configured to attach a test specimen, an imaging device configured to capture images of a surface of the test specimen when attached in the load string, a storage device configured to store a plurality of first calibration parameters corresponding to intrinsic characteristics of the imaging device, and a control circuit configured to perform a calibration process using the first calibration parameters to calculate a plurality of second calibration parameters based on a positioning of the test specimen relative to the imaging device, and perform an optical extensometer process to measure displacement of the test specimen based on the first calibration parameters and the second calibration parameters.

[0042] In some example optical extensometers, the imaging device includes a housing, an image sensor within the housing, and a lens configured to be mounted to the housing using a kinematic mount configured to replicate a position and orientation of the lens corresponding to a calibration procedure associated with generating the first calibration parameters. In some example optical extensometers, the kinematic mount includes a base having a plurality of seats and a plurality of first magnets, a lens mount having a plurality of bearings having complementary arrangements to the plurality of seats, and a plurality of second magnets configured to interact with the first magnets to mate the plurality of bearings with corresponding ones of the plurality of seats.

[0043] In some exemplary optical extensometers, the control circuitry is configured to calculate the second calibration parameters based on a first calibration plate having a different set of visual features than a second calibration plate used to determine and store the first calibration parameters. In some exemplary optical extensometers, the intrinsic characteristics of the imaging device include one or more of a focal length, an optical center, a distortion parameter, a pixel size, or a pixel skew parameter. In some exemplary optical extensometers, the second calibration parameters include a relative position of the imaging device and the load string with respect to a reference position and a relative rotation of the imaging device and the load string with respect to a reference orientation.

[0044] Referring now to the figures, Figure 1 illustrates an exemplary optical extensometer system 10 for measuring changes in one or more properties of a test specimen 16 undergoing mechanical property testing. The exemplary extensometer system 10 can be connected to a testing system 33 (e.g., a load string) capable of mechanically stressing the test specimen 16, for example. The extensometer system 10 can measure and / or calculate changes in the test specimen 16 undergoing, for example, compressive strength testing, tensile strength testing, shear strength testing, bending strength testing, flexure strength testing, tear strength testing, peel strength testing (e.g., adhesive bond strength), torsional strength testing, and / or any other compressive and / or tensile testing. Additionally or alternatively, the extensometer system 10 can perform dynamic testing.

[0045] According to a disclosed example, the extensometer system 10 may include a testing system 33 that manipulates and tests the test specimen 16, and / or a computing device 32 (e.g., a processing system) communicatively coupled to the testing system 33, a light source, and / or an imaging device, as further shown in Figure 4. The testing system 33 applies a load to the test specimen 16 and measures a mechanical property of the test, such as the displacement of the test specimen 16 and / or the force applied to the test specimen 16.

[0046] The extensometer system 10 includes a remote and / or integrated light source 14 (e.g., an LED array) and / or a reflective back screen 18 that illuminates the test specimen 16. The extensometer system 10 includes a computing device 32 (see also FIG. 4) and a camera or imaging device 12. While the example of FIG. 1 shows an imaging device 12 having a single camera, the disclosed examples are applicable to multi-camera extensometer systems 10. In some examples, the light source 14 and imaging device 12 are configured to transmit and receive at one or more desired wavelengths (e.g., visible wavelengths, infrared wavelengths, ultraviolet wavelengths, etc.), although other illumination sources and / or wavelengths are applicable as well. In some examples, one or both of the light source 14 or imaging device 12 include one or more filters (e.g., polarizing filters), one or more lenses. In some examples, a calibration routine (e.g., a two-dimensional calibration routine) is performed to identify one or more characteristics of the test specimen 16, one or more markers 20 (including a pattern of markers), and / or other characteristics of the test specimen 16, and / or other characteristics of the markers 20 (including a pattern of markers), ...

[0047] In the disclosed example, the computing device 32 can be used to configure the test system 33, control the test system 33, and / or receive measurement data (e.g., transducer measurements such as force and displacement) and / or test results (e.g., peak force, break displacement, etc.) from the test system 33 for processing, displaying, reporting, and / or any other desired purpose. The extensometer system 10 connects to the test system 33 and software utilizing any standard interface, such as USB 1.0, USB 1.1, USB 2.0, USB 3.0, Ethernet, analog, encoder, or SPI, and / or any other standard and / or custom interface. The use of standard interfaces allows the extensometer system 10 to be plugged into and used by existing systems without the need for special integrated software or hardware. The extensometer system 10 provides real-time axial and lateral encoder or analog information to the test system 33. The real-time optical extensometer 10 and materials testing machine 33 exchange real-time test data, including extension / strain data, with a computing device 32, which may be configured via wired and / or wireless communication channels. The extensometer system 10 provides measurements and / or calculations of the extension / strain data captured from the test specimen 16 undergoing testing in the testing system 33, and further provides stress data and extension / strain data to the computing device 32.

[0048] As disclosed herein, the captured image is input from the imaging device to a computing device 32. The processor uses one or more algorithms and / or look-up tables to calculate multi-axial stretch / strain values ​​of the test specimen 16 (i.e., the change or percentage change in target-to-target distance calculated by image monitoring of the markers 20 affixed to the test specimen 16). After calculation, this data may be stored in memory or output to a network and / or one or more display devices, I / O devices, etc. (see also FIG. 4).

[0049] FIG. 2 is an exemplary test specimen 16 that may be measured in the extensometer system 10 of FIG. 1. For example, one or more markings 20 (e.g., fiducial features) are affixed to a surface 28 that faces the light source 14 and imaging device 12. The grippers 26 are configured to be placed in grips of a test system 33 (see also FIG. 4) to apply a force to the test specimen 16. For example, a cross-member loader applies a force to the test specimen 16 while the grippers are gripping or otherwise coupled to the test system 33. A force applicator, such as a motor, moves the crosshead relative to the frame to apply a force to the test specimen 16 as indicated by a double-headed arrow 34. The force 34 that pulls the grippers 26 apart may stretch the test specimen 16, causing the markings to move from a first position 20A to a second position 20B. Additionally or alternatively, the markings may change shape or size, which can also be measured by the computing device 32 by viewing the captured image. The force 34 can also move the edge of the test specimen from the first position 22A to the second position 22B. For example, in the first or initial position, the edge has a width 24A and decreases to width 24B upon application of the force 34.

[0050] Based on the captured images, the computing device 32 is configured to perform stretching / straining in the measurement process. For example, to detect stretching / straining in the test specimen 16, the computing device 32 monitors the images provided via the imaging device 12. When the computing device 32 identifies a change in the relative position between two or more of the markers and / or edges of the test specimen 16 (e.g., compared to an initial location at the beginning of the crosshead movement), the computing device 32 measures the amount of change and calculates the amount of stretching and / or strain in the test specimen 16. As disclosed herein, the markers are configured to reflect light from a light source to the camera, while the back screen reflects the light to generate a dark silhouette for edge analysis.

[0051] As disclosed herein, optical extensometer system 10 is configured to make optical width measurements of an opaque test specimen 16. An imaging device 12 is positioned to view a surface 28 of the test specimen 16 that faces the imaging device 12. Surface 28 is located near the focal plane of the imaging device optics (see, e.g., FIG. 3).

[0052] As shown in Figure 3, the optical extensometer system 10 is arranged to measure one or both of axial strain (based on changes in the markers 20 and / or marker pattern on the front surface 28 of the test specimen 16) and transverse strain (calculated from changes in the width of the specimen 16). The components of the optical extensometer system 10 are shown in a top perspective view in Figure 3, where the location of each component relative to the other components is schematic. As shown, the components include an imaging device 12 (e.g., a video camera) configured to capture one or more images of the test specimen 16 during physical testing (e.g., at regular intervals, continuously, and / or based on one or more thresholds associated with time, force, or other suitable test characteristic).

[0053] As shown, the imaging device 12 and the test sample 16 are positioned at a working distance or Z-axis distance 39, which may be static, predetermined, and / or variable during the testing process.

[0054] The test specimen 16 is characterized by suitable marks or fiducial features 20 on a forward facing surface 28 (and / or an opposing surface) of the test specimen 16. Analysis of one or more images associated with the optical extensometer system 10 is performed via a computing device 32 to execute an identification algorithm that allows both the markings 20 of the test specimen 16 and the edges 22 of the test specimen to be continuously tracked and measured during the testing process.

[0055] In the illustrated example, imaging device 12 is a single view camera having a single optical axis 50. In some examples, two or more imaging devices may be employed, which may be arranged and / or positioned at different viewing angles of test sample 16. By employing a stereo imaging configuration, measurement variables related to perspective and / or depth of multiple dimensions of test sample 16 may also be used to further calibrate and / or measure properties of test sample 16.

[0056] In some examples, the optical extensometer system 10 can measure Z-axis movement by analyzing changes associated with features of the test sample that are independent of deformation of the test sample. For example, an image or other feature may be projected onto the surface of the test sample. For example, a laser and / or other type of projector may project a feature (e.g., a point, line, pattern, etc.). The imaging device 12 can measure Z-axis movement by measuring the change and / or displacement of the projected feature, such as by using a known angle α between the projected light and the surface of the test sample.

[0057] In some examples, the measurements and / or positions of the one or more edges are provided in pixel coordinates captured by imaging device 12. Additionally or alternatively, the measurements and / or positions of the one or more edges are provided in other standard coordinate systems / units, such as meters. In such examples, a calibration process may be implemented to determine the absolute and / or relative positioning and / or dimensions of the test specimen within the test system prior to measurements, and similar coordinate systems / units may be employed during the testing process.

[0058] FIG. 4 is a block diagram of an exemplary extensometer system 10 of FIG. 1. As shown in FIG. 1, the extensometer system 10 includes a test system 33 and a computing device 32. The exemplary computing device 32 can be a general purpose computer, a laptop computer, a tablet computer, a mobile device, a server, an all-in-one computer, and / or any other type of computing device. The computing device 32 of FIG. 4 includes a processor 202, which can be a general purpose central processing unit (CPU). In some examples, the processor 202 can include one or more dedicated processing devices, such as an FPGA, a RISC processor with an ARM core, an image processing device, a digital signal processor, and / or a system on a chip (SoC). The processor 202 executes machine-readable instructions 204, which can be stored locally at the processor (e.g., in an internal cache or on the SoC), in a random access memory 206 (or other volatile memory), in a read-only memory 208 (or other non-volatile memory, such as a flash memory), and / or in a mass storage device 210. The exemplary mass storage device 210 may be a hard drive, a solid state storage drive, a hybrid drive, a RAID array, and / or any other mass data storage device. The bus 212 enables communication between the processor 202, the RAM 206, the ROM 208, the mass storage device 210, the network interface 214, and / or the input / output interface 216.

[0059] An example network interface 214 includes hardware, firmware, and / or software that connects computing device 32 to a communications network 218, such as the Internet. For example, network interface 214 may include IEEE 202.X compliant wireless and / or wired communications hardware for transmitting and / or receiving communications.

[0060] The example I / O interface 216 of FIG. 4 includes hardware, firmware, and / or software that couples one or more input / output devices 220 to the processor 202 to provide input to and / or output from the processor 202. For example, the I / O interface 216 can include an image processing device that interfaces with a display device, a Universal Serial Bus port that interfaces with one or more USB-compliant devices, FireWire, Fieldbus, and / or any other type of interface. The example extensometer system 10 includes a display device 224 (e.g., an LCD screen) coupled to the I / O interface 216. Other example I / O device(s) 220 can include a keyboard, a keypad, a mouse, a trackball, a pointing device, a microphone, an audio speaker, a display device, an optical media drive, a multi-touch touch screen, a gesture recognition interface, a magnetic media drive, and / or any other type of input and / or output device.

[0061] Computing device 32 may access non-transitory machine-readable medium 222 via I / O interface 216 and / or I / O device(s) 220. Examples of machine-readable medium 222 in Figure 4 include optical disks (e.g., compact disks (CDs), digital versatile / video disks (DVDs), Blu-ray® disks, etc.), magnetic media (e.g., floppy disks), portable storage media (e.g., portable flash drives, secure digital (SD) cards, etc.), and / or any other type of removable and / or installed machine-readable medium.

[0062] The extensometer system 10 further comprises a test system 33 coupled to the computing device 32. In the example of Figure 4, the test system 33 is coupled to the computing device via an I / O interface 216, such as a USB port, a Thunderbolt port, a FireWire (IEEE 1394) port, and / or any other type of serial or parallel data port. In some examples, the test system 33 is coupled to the network interface 214 and / or the I / O interface 216 via a wired or wireless connection (e.g., Ethernet, Wi-Fi, etc.), either directly or via a network 218.

[0063] Test system 33 includes a frame 228, load cells 230, displacement transducers 232, cross member loader 234, material fixture 236, and control processor 238. Frame 228 provides rigid structural support for the other components of test system 33 that perform the tests. Load cells 230 measure the force applied by cross member loader 234 through grips 248 to the material under test. Test system 33 may include any other type of transducers for measuring force, displacement, strain, and / or any other desired variable.

[0064] The cross-member loader 234 applies a force to the material under test while a material fixture 236 (also referred to as a grip) grips or otherwise couples the material under test to the cross-member loader 234. An example cross-member loader 234 includes a motor 242 (or other actuator) and a crosshead 244. As used herein, "crosshead" refers to a component of a materials testing system that applies directional (axial) and / or rotational forces to a specimen. A materials testing system can have one or more crossheads, and the crosshead(s) can be positioned in any suitable position and / or orientation in the materials testing system. The crosshead 244 couples the material fixture 236 to the frame 228, and the motor 242 moves the crosshead relative to the frame to position the material fixture 236 and / or apply forces to the material under test. Exemplary actuators that may be used to provide force and / or movement of the components of the extensometer system 10 include electric motors, pneumatic actuators, hydraulic actuators, piezoelectric actuators, relays, and / or switches.

[0065] Although the example test system 33 uses a motor 242, such as a servo motor or a direct drive linear motor, other systems may use different types of actuators, for example hydraulic actuators, pneumatic actuators, and / or any other type of actuator based on the requirements of the system.

[0066] Exemplary grips 236 include compression platens, jaws, or other types of fixtures depending on the mechanical property and / or material being tested. Grips 236 can be manually configured, controlled via manual input, and / or automatically controlled by control processor 238. Crosshead 244 and grips 236 are operator accessible components.

[0067] The extensometer system 10 may further include one or more control panels 250 including one or more mode switches 252. The mode switches 252 may include buttons, switches, and / or other input devices located on an operator control panel. For example, the mode switches 252 may include a button that controls the motor 242 to jog (e.g., position) the crosshead 244 to a particular position on the frame 228, a switch (e.g., a foot switch) that controls the grip actuator 246 to open or close the pneumatic grips 248, and / or any other input device that controls the operation of the test system 33.

[0068] The example control processor 238 communicates with the computing device 32, e.g., receives test parameters from the computing device 32 and / or reports measurements and / or other results to the computing device 32. For example, the control processor 238 may include one or more communication or I / O interfaces that enable communication with the computing device 32. The control processor 238 may control the cross member loader 234 to increase or decrease the applied force, may control the fixture(s) 236 to grip or release the material under test, and / or may receive measurements from the displacement transducers 232, the load cells 230, and / or other transducers.

[0069] The example control processor 238 is configured to perform the stretch / strain measurement process as the test specimen 16 is undergoing testing in the testing system 33. For example, to detect stretch / strain in the test specimen 16, the control processor 238 monitors images provided via the imaging device 12. Once the control processor 238 identifies a change in location and / or position of the edge 22 of the test specimen 16 (e.g., compared to an initial location at the beginning of the crosshead 244 movement), the control processor 238 measures the amount of change to calculate the amount of stretch and / or strain in the test specimen 16. For example, real-time video provided by the imaging device 12 captures the absolute position of the edge 22 and monitors their relative movement over several images to calculate the stretch / strain in real time. Stress and strain data is exchanged between the real-time optical extensometer 10, the testing system 33 and the computing device 32 and is typically organized and displayed via the display device 224.

[0070] Conventional video extensometer systems require calibration by an operator to control for intrinsic and / or extrinsic variables that may affect video extensometer measurements. In some conventional systems, an operator is encouraged or required to recalibrate the video extensometer in response to certain (or any) changes in the system, such as changing lenses, changing focus, or changing positioning. In some such systems, calibration can be time-consuming and / or tedious by requiring installation of a specific calibration plate on the load string. In other conventional video extensometer systems, the video extensometer is calibrated for a specific working distance and may emit a visual aid, such as a visible pattern, to allow the operator to visually identify when the calibrated working distance has been achieved. However, such visual aids may be dependent on the operator correctly interpreting the visual aid to constitute the calibrated working distance.

[0071] As described in more detail below, the disclosed examples of the optical extensometer 10 reduce or eliminate the calibration burden on the operator by calibrating all or a portion of the required parameters during manufacture and maintaining the optical extensometer 10 in a calibrated state after delivery. Factors for which the optical extensometer 10 may be calibrated include intrinsic properties of the optical extensometer 10 and extrinsic properties related to the configuration of the optical extensometer 10. Exemplary intrinsic properties include the focal length of the imaging device 12, the optical center of the imaging device 12, the distortion parameter(s), the pixel size(s) of the imaging device 12, and / or a parameter(s) representing pixel skew in the imaging device 12. Exemplary extrinsic properties include the relative position (e.g., X, Y, Z position) of the imaging device 12 and the load string with respect to a reference position, and the relative rotation of the imaging device 12 and the load string with respect to a reference orientation.

[0072] In some examples, the split calibration approach involves calibrating parameters corresponding to the intrinsic properties of the optical extensometer 10 prior to delivery to an operator (e.g., at a factory, by a manufacturer or reseller, etc.) and storing the calibrated parameters in a storage device (e.g., RAM 206, ROM 208, mass storage device 210, machine-readable medium 222, etc.) of the optical extensometer 10. After the optical extensometer 10 is delivered and installed to an operator, the extrinsic properties can be readily calibrated by the operator.

[0073] Upon installation and configuration, the exemplary optical extensometer 10 allows for the use of a verification plate that is simpler, smaller, and easier to use than conventional calibration plates. Because the intrinsic calibration parameters are already stored in the optical extensometer 10 prior to installation, the intrinsic calibration parameters can be used to determine the extrinsic calibration parameters using a simpler and / or smaller verification plate.

[0074] 5A illustrates an exemplary arrangement of an imaging device 12 and a calibration plate 502 that may be used to calibrate an optical extensometer system 10 for intrinsic properties and store calibration parameters. The exemplary imaging device 12 is positioned at a known working distance 504 from the calibration plate 502. The calibration process illustrated in FIG. 5A is performed prior to delivery and installation of the optical extensometer 10. The calibration process may be similar to a conventional calibration process, such as observing a pattern of markers with known spacing and size on the calibration plate 502 and calculating appropriate parameters to compensate for the focal length, optical center, distortion, pixel size, pixel skew, and / or any other aspect of the imaging device 12.

[0075] In some other examples, parameters for both the intrinsic and extrinsic properties may be calibrated (e.g., at a factory, by a manufacturer or reseller, etc.) prior to delivery to an operator, and the calibrated parameters are stored in a storage device. The parameters may be calibrated for a number of predefined working distances. After the optical extensometer 10 is delivered to an operator and installed, the installer or operator may adjust the imaging device 12 to verify that the working distance is properly configured (e.g., within a selected calibrated working distance threshold range) and make adjustments to the position and / or orientation of the imaging device 12 to correct for deviations in position and / or orientation from the calibrated configuration. After calculating the calibration parameters (e.g., via the processor 202), the example processor 202 stores the calibration parameters (e.g., in the mass storage device 210). In some examples, the stored calibration parameters are directly related to the optics (e.g., lenses, lens mounting devices) and / or other specific components of the imaging device 12 to ensure that the stored calibration parameters are accurate at a later point in time.

[0076] FIG. 5B illustrates an exemplary arrangement of an imaging device 12 and a calibration plate 502 that can be used to calibrate the optical extensometer system 10 for intrinsic and extrinsic properties at a number of predefined working distances 506a-506d and store the calibration parameters. The exemplary imaging device 12 is positioned at each of the working distances 506a-506d from the calibration plate 502. The calibration process illustrated in FIG. 5B is performed prior to delivery and installation of the optical extensometer 10. The calibration process may be similar to a conventional calibration process, such as observing a pattern of markers with known spacing and size on the calibration plate 502 and calculating appropriate parameters to compensate for extrinsic parameters such as focal length, optical center, distortion, pixel size, pixel skew, and / or any other aspect of the imaging device 12 as well as the working distances 506a-506d.

[0077] In each of the example calibration processes of FIGS. 5A and 5B, the calibration plate 502 is placed into a load string 508, which may include the material fixture 236, the load cell 230, and / or the frame 228 of FIG. 2, and / or any other components involved in applying and / or measuring loads on a sample during testing.

[0078] 6A illustrates an example arrangement of an imaging device 12 and a verification plate 602 that can be used to verify and adjust calibration parameters associated with a configured working distance 604. The example imaging device 12 is shown in an installed configuration in FIG. 6A with the verification plate 602 installed in a load string 508 that is configured during installation to be the configured working distance 604 from the imaging device 12. In some other examples, rather than installing the verification plate 602 directly in the load string 508, the verification plate 602 may be attached to a specimen or other object installed in the load string 508. For example, the verification plate 602 may be clamped or clipped (e.g., using a spring clip or other type of clamp or clip) to a specimen installed in the load string 508. In some such examples, the verification plate 602 may include or be attached to a centering device to provide a desired positioning of the verification plate 602 within the field of view of the imaging device 12.

[0079] Because the intrinsic calibration parameters are pre-calculated prior to installation, the exemplary validation plate 602 used to determine the extrinsic calibration parameters may be smaller and / or simpler than the calibration plate 502 used to perform the calibration of the intrinsic calibration parameters. For example, the validation plate 602 may be smaller and / or include a fewer number of markers 606 than the calibration plate 502. The markers 606 have a precise predetermined spatial relationship and size to enable the processor 202 to measure (e.g., via the imaging device 12) and calculate the working distance 604, any other position offsets, and / or relative rotation between the imaging device 12 and the load string 508. The validation plate 602 has a limited number of markers 606 due to the limited use of the validation plate 602 relative to the calibration plate 502, where the calibration plate 502 includes more markers to enable a fuller calibration of the intrinsic aspects of the imaging device's optics.

[0080] 6A shows one example verification plate 602, other verification plate designs may be used. For example, the size of the verification plate, the size(s), shape(s), and / or position(s) of the markers, and / or any other aspect of the verification plate may be modified for a desired application. Additionally or alternatively, the verification process may be performed using features projected (e.g., via one or more lasers) onto the test sample or other surface coplanar with the load string 508, in which case the imaging device 12 uses a known projection angle to detect the distance between the projected feature(s) and / or an object having a known position to determine the working distance 604.

[0081] The processor 202 uses the intrinsic calibration parameters stored in the mass storage device 210 to measure and calculate extrinsic calibration parameters, such as determining the actual working distance 604 between the imaging device 12 and the load string 508 (e.g., the plane of the specimen being tested via the load string). The processor 202 can then store the measured and calibrated extrinsic parameters in the mass storage device 210 (or other storage device of the optical extensometer 10) for use during the measurement process.

[0082] Figure 6B illustrates another example arrangement of the imaging device 12 of Figure 5B and a load string 610 having a verification marker 612 that can be used to verify and adjust calibration parameters associated with a configured one of the predetermined working distances. In contrast to the example of Figure 6A, the verification marker 612 is positioned on the load string 610, such as on the grip 248, instead of on a removable verification plate. As a result, the verification marker 612 can remain on the load string 610 and within the field of view of the imaging device 12 between and between measurement operations.

[0083] 6C illustrates another example arrangement of the imaging device 12 of FIG. 5B and a reference tab 620 having a verification marker 622 that can be used to verify and adjust calibration parameters associated with a configured one of the predetermined working distances. The example reference tab 620 is installed into the load string 610 such that the face of the reference tab 620 having the marker 622 is flush with the specimen 624 being tested. The markers 622 are configured to have a predetermined spacing and size(s) so that the imaging device 12 can determine and / or monitor (e.g., in real time) the working distance 604 and / or any rotation and / or movement of the load string 610 during testing. One or more reference tabs 620 having markers 622 of any desired number and / or arrangement can be used.

[0084] To enable the stored calibration parameters for the intrinsic characteristics to remain valid from the calibration procedure through delivery and installation, the disclosed example imaging device 12 includes a kinematic mount for mounting and dismounting one or more lenses to and from the imaging device 12 in a consistent and repeatable manner. The kinematic mount is self-mounting and self-positioning, and is not subject to variations in skill or technique between operators. By using the kinematic mount, the example imaging device 12 enables repeatability of the exact position and orientation of the lenses to ensure the validity of the calibrated intrinsic parameters.

[0085] FIG. 7 is a plan view of an example base 702 for a kinematic mounting system for a lens assembly of the example imaging device 12 of FIG. 1. The example base 702 is rigidly attached to the housing of the imaging device 12 such that the base 702 has a fixed position relative to the optical sensor of the imaging device 12 and / or relative to the housing. FIG. 8 is a plan view of an example lens mount 802 for a kinematic mounting system for a lens assembly configured to connect to the base of FIG. 7 in a precise and repeatable manner. A lens is attached to the lens mount 802 to form a lens assembly, which is removable and replaceable in the imaging device 12. Additionally or alternatively, the lens mount 802 may allow for the attachment of optical filters, polarizers, and / or any other optical components for use with the optical sensor. FIG. 9 is a perspective view of the example lens mount 802 of FIG. 8.

[0086] The base 702 includes a set of fixed seats 704a, 704b, 704c and a set of magnets 706a, 706b, 706c. The illustrated seats 704a, 704b, 704c are spaced apart and each includes a set of parallel dowel pins 708a, 708b, 708c. In the example, the seats 704a, 704b, 704c are spaced apart at 120 degree intervals, but may be spaced apart in any other desired configuration that supports the lens mount 802. On the opposite side of the base 702, the lens mount 802 includes a set of bearings 804a, 804b, 804c and a set of magnets 806a, 806b, 806c. The magnets 706a, 706b, 706c and magnets 806a, 806b, 806c provide at least a threshold mounting or clamping force to seat the bearings 804a, 804b, 804c in the seats 704a, 704b, 704c. The example bearings 804a, 804b, 804c have a complementary arrangement with the seats 704a, 704b, 704c such that each of the bearings 804a, 804b, 804c mates with a corresponding one of the seats 704a, 704b, 704c during installation. Figure 10 is a cross-sectional view of the lens mount 802 and base 702 of the example imaging assembly of Figures 7-9, illustrating the mounting of the bearing 804a of the lens mount 802 in the seat 704a of the base 702.

[0087] The base 702 and lens mount 802 may include a keying mechanism that prevents the bearings 804a, 804b, 804c from being installed in one of the seats 704a, 704b, 704c differently than the orientation at the time the calibration was performed. For example, the keying mechanism may include a dowel 810 and corresponding slot 710 positioned to only allow installation in a single angled orientation. In some examples, the keying mechanism 710, 810 provides sufficient clearance to allow the bearings 804a, 804b, 804c to be properly and fully seated into the seats 704a, 704b, 704c.

[0088] The example lens mount 802 further includes a skirt 812 that reduces or blocks stray light from entering between the lens mount 802 and the base 702. In some other examples, gaskets, O-rings, and / or any other light blocking techniques may be used.

[0089] Depending on the materials used to implement the base 702 and the lens mount 802, one of the base 702 or the lens mount 802 may omit an associated magnet. In some other examples, the magnet may be replaced with other features or devices to provide the desired mounting force between the base 702 and the lens mount 802. For example, for a vertically mounted lens mount 802 with sufficient mass, gravity may be utilized. In other examples, screws, springs, and / or other fastening devices may be used to attach the lens mount 802 to the base 702 with sufficient clamping force.

[0090] In the illustrated example of Figures 7 and 8, base 702 includes seats 704a, 704b, and 704c, and lens mount 802 includes bearings 804a, 804b, and 804c, however, in other examples, base 702 may be configured with bearings configured to engage corresponding seats on the lens mount.

[0091] In some examples, the lens mount 802 may include wireless communication methods, such as a radio frequency identifier (RFID) or near field communication (NFC) tag, or a wired communication device, such as a secure data storage chip or programmable read only memory (PROM), which may be readable by a corresponding reader on or near the base 702. In some examples, the bearings 804a-804c and dowel pins 708a-708c may provide electrical contacts for communication between a processor (e.g., processor 202) and a data storage device on the lens mount 802, such that the processor is communicatively connected to the data storage device when the lens assembly is attached to the imaging device 12.

[0092] The data storage device on the lens mount 802 may include a serial number or other identifier that is securely read and stored along with the first calibration parameters. In other examples, the user interface of the optical extensometer 10, a remote control system (e.g., a smartphone app, a web interface, etc.), and / or any other user interface system may require an operator to input a serial number or identifier that is printed, engraved, or otherwise marked on the lens assembly. In other examples, the identifier may be input into the optical extensometer 10 optically (e.g., via an optical sensor configured to read observable indicia on the lens mount 802 during and / or after installation of the lens mount 802), mechanically, electrically (e.g., via a unique resistor array for each unique optical sensor), and / or using any other technique.

[0093] At a time subsequent to the intrinsic calibration, the data storage tag on the lens mount 802 may be read and compared to the identifier associated with the intrinsic parameters to verify that the calibration parameters stored for the installed lens assembly are valid. If an incorrect lens assembly is installed (e.g., the entered identifier does not match the identifier associated with the calibration parameters), the optical extensometer 10 will identify the discrepancy between the identifiers and indicate that a new calibration process is required to ensure a valid calibration.

[0094] Additionally or alternatively, the lens mount 802 may include visually identifiable markings that may be identified by the processor 202 via images captured via an optical sensor of the imaging device 12. For example, any portion of the body 802 may include a barcode, a QR code, and / or any other visually identifiable marking. The markings may include or represent an encoded identifier and / or any other desired information and may be placed within the field of view of the imaging device 12 during or prior to installation of the lens mount 802, at which point the processor 202 reads the markings in a manner similar to that described above and compares the observed identifier to stored identifiers associated with stored calibration parameters.

[0095] The kinematic mount shown in Figures 7-10 ensures that the orientation of the lens relative to the optical sensor is constant, which ensures that the lens characteristics (e.g., distortion) always remain in the same orientation relative to the optical sensor during the calibration process to determine the intrinsic calibration parameters. In addition, the bearing mounting on the dowel pins provides a precise and repeatable axial location of the lens to consistently maintain the focus of the lens when the lens assembly is installed.

[0096] Although one exemplary kinematic mount is shown in Figures 7-10, other kinematic mounts may be used. For example, the quantity, positioning, and / or type of seats and bearings may be modified. Similarly, the quantity, positioning, and / or type of keying features may be modified. For example, instead of dowels and slots for keying, magnet placement may be selected such that only one orientation results in engagement of the magnets. In other examples, the keying features may include asymmetries in the base 702 and lens mount 802.

[0097] The lens mount 802 may include one or more features that allow an operator to grip the lens mount 802 for installation without touching the attached lens. In some examples, the base 702 and / or the lens mount 802 include heat dissipation fins or other heat dissipation features. The lens mount 802 and base 702 may be constructed using any desired material, such as metal, plastic, and / or ceramic.

[0098] 11A-11C show an exemplary progression of repeatable installation of the lens mount 802 of FIG. 8 onto the base 702 of FIG. 7. As shown in FIG. 11A, the lens mount 802 is moved towards the base 702. The lens mount 802 may require rotation (FIG. 11B) so that the keying features 710, 810 allow for full installation of the lens mount 802 onto the base 702. Once the keying features 710, 810 are mated and the lens mount 802 is within a threshold distance of the base 702, the opposing magnets 706a-706c, 806a-806c create sufficient attractive force to mate the lens mount 802 with the base 702 and seat the bearings 804a-804c into their corresponding ones of the seats 704a-704c. The magnets 706a-706c, 806a-806c provide a consistent mating force between the bearings 804a-804c and the seats 704a-704c to provide a consistent seating or mounting depth of the bearings 804a-804c into the seats 704a-704c that is not affected by the skill or ability of the operator.

[0099] 12A illustrates an example of an imaging device 12 including an adjustable imaging device mount 1202 mounted inside a housing 1204 for adjusting the positioning and / or rotation of an optical sensor 1206 and a lens assembly 1208 of the imaging device 12. The housing 1204 includes the optical sensor 1206 of the imaging device 12, and the lens assembly 1208 is mounted to the housing 1204. The lens assembly 1208 may be implemented by the example base 702 and lens mount 802 of FIGS. 7-10. The lens assembly 1208 has a fixed distance relative to the optical sensor 1206 when installed (e.g., for the validity of calibrated parameters).

[0100] The housing 1204 is physically attached to a support structure, such as a support rail 1210. The support rail 1210 has a fixed position relative to the load string of the optical extensometer 10. As a result, the housing 1204 also has a fixed position relative to the load string. The example adjustable imaging device mount 1202 is attached between the optical sensor 1206 and the housing 1204 and allows for manual (or automatic) adjustment of the physical position and / or orientation of the optical sensor 1206 and lens assembly 1208 relative to the load string, which allows for adjustment of the working distance (e.g., working distance 604 of FIG. 6A), particularly during an extrinsic validation process in which a calibrated working distance (e.g., one of the multiple working distances 506a-506d) is configured and validated. The adjustable imaging device mount 1202 may include, for example, an adjustment screw (e.g., a jackscrew) that may be manipulated to adjust the working distance and / or other positioning and / or rotational aspects.

[0101] 12B illustrates another example of an imaging device 12 that includes an adjustable imaging device mount 1212 attached to the exterior of the housing 1204 for adjusting the positioning and / or rotation of the optical sensor 1206 and lens assembly 1208 of the imaging device 12. In the example of FIG. 12B, the adjustable imaging device mount 1212 is connected between the housing 1204 and a support rail 1210 (or other support structure). As a result, the adjustable imaging device mount 1212 is configured to adjust the position (e.g., working distance) and / or orientation of the optical sensor 1206 relative to the load string by adjusting the housing 1204.

[0102] The example adjustable imaging device mount 1212 allows for manual (or automatic) adjustment of the physical position and / or orientation of the optical sensor 1206 and lens assembly 1208 relative to the load string, which allows for adjustment of the working distance (e.g., working distance 604 in FIG. 6A ), particularly during an extrinsic validation process in which a calibrated working distance (e.g., one of the multiple working distances 506a-506d) is constructed and validated. The adjustable imaging device mount 1212 may include, for example, an adjustment screw (e.g., a jackscrew) that can be manipulated to adjust the working distance and / or other positioning and / or rotational aspects.

[0103] In some other examples, adjustments of the working distance, focal length, and / or other positions or orientations may be made using spacers or other techniques to effect physical adjustments. In some examples, the adjustments may be performed by a dial, where turning the dial moves the optical sensor or base 702 of the lens assembly 1208 along a thread. In some examples, the dial may include multiple detents that allow an operator to dial the lens assembly 1208, base 702, or optical sensor to one of multiple discrete adjustment points. As an example, the base 702 may be dialed to a desired distance from the optical sensor to adjust the focal length for a desired working distance.

[0104] Figure 13 is a flow diagram depicting one example method 1300 that may be performed to determine and store intrinsic calibration parameters for the optical extensometer system 10 of Figure 1. The example method 1300 is performed prior to delivery and installation of the optical extensometer system 10 to determine at least the intrinsic calibration parameters. The method 1300 is described below with reference to the example optical extensometer 10 of Figures 5A and 6A, but may also be performed using other optical extensometer systems.

[0105] At block 1302, an operator assembles the optical extensometer 10, including a load string (e.g., load string 508 of FIG. 5A), an imaging device (e.g., imaging device 12 at a working distance 504 from the load string 508), and a computing system (e.g., computing device 32 of FIG. 4). Assembly of the imaging device 12 may include, for example, mounting a lens assembly on a base using a kinematic mount. Assembly may further include mounting a calibration plate (e.g., calibration plate 502 of FIG. 5A) on the load string 508.

[0106] At block 1304, the optical extensometer 10 calibrates (e.g., via the processor 202 of FIG. 5A ) first calibration parameters corresponding to intrinsic properties of the imaging device 12 using the calibration plate 502 and stores the first calibration parameters in a storage device (e.g., the mass storage device 210). For example, the processor 202 may determine and store calibration parameters for one or more of the focal length of the imaging device 12, the optical center of the imaging device 12, distortion parameter(s), pixel size(s) of the imaging device 12, and / or parameter(s) representing pixel skew in the imaging device 12.

[0107] At block 1306, the optical extensometer 10 is delivered to a user. The delivered optical extensometer 10 includes an imaging device 12 that includes the same lens assembly that was used during calibration, and a computing device 32 that includes a storage device that includes the calibration parameters. In some examples, the delivered optical extensometer 10 also includes the load string that was used during calibration. In other examples, the load string may be different in the delivered system than during calibration. The example method 1300 then ends.

[0108] Upon delivery, the optical extensometer 10 is installed and further calibrated using the pre-calibrated intrinsic parameters. Figure 14 is a flow diagram depicting an example method 1400 that may be performed to determine a second extrinsic calibration parameter for the optical extensometer system 10 of Figure 1 based on the first intrinsic calibration parameter. Method 1400 is described below with reference to the example optical extensometer 10 of Figures 5A and 6A, but may also be performed using other optical extensometer systems.

[0109] At block 1402, the optical extensometer 10, including the load string 508, the imaging device 12, and the computing device 32, are positioned in a desired arrangement. For example, the imaging device 12 and the load string 508 may be positioned at a desired working distance 604.

[0110] At block 1404, the optical extensometer 10 calibrates (e.g., via the processor 202) a second calibration parameter corresponding to the placement of the installed optical extensometer 10 using a second calibration plate (e.g., the validation plate 602 of FIG. 6A) and the first calibration parameters stored in the storage device. The processor 202 stores the second calibration parameter. In the example method 1400, the second calibration parameter relates to exogenous factors, such as the particular placed working distance 604, any other positional offset between the imaging device 12 and the load string 508, and / or any rotation between the imaging device 12 and the load string 508.

[0111] At block 1406, the operator may perform an optical deformation or distortion measurement process using the first calibration parameters and the second calibration parameters.

[0112] In block 1408, the operator or the processor 202 may determine whether a change has occurred in the placement of the installed optical extensometer 10. For example, the processor 202 may identify one or more factors that indicate that the working distance 604 (e.g., via monitoring a reference tab or similar device) or other aspect of the system has changed, or the operator may indicate via a user interface that such a change has occurred.

[0113] If no change in configuration has been detected (block 1408), control returns to block 1406 to continue performing the measurement process. If a change in configuration has occurred (block 1408), control returns to block 1404 to again perform the calibration of the second calibration parameter.

[0114] Figure 15 is a flow diagram depicting one example method 1500 that may be performed to determine and store intrinsic and extrinsic calibration parameters for the extensometer system of Figure 1. The example method 1300 is performed prior to delivery and installation of the optical extensometer system 10 to determine at least the intrinsic calibration parameters. The method 1500 is described below with reference to the example optical extensometer 10 of Figures 5B and 6A, but may also be performed using other optical extensometer systems.

[0115] At block 1502, an operator assembles the optical extensometer 10, including a load string (e.g., load string 508 of FIG. 5B), an imaging device (e.g., imaging device 12 at a working distance 506a-506d from the load string 508), and a computing system (e.g., computing device 32 of FIG. 4). Assembling the imaging device 12 may include, for example, mounting a lens assembly on a base using a kinematic mount. Assembly may further include mounting a calibration plate (e.g., calibration plate 502 of FIG. 5B) on the load string 508.

[0116] At block 1504, the optical extensometer 10 calibrates (e.g., via the processor 202 of FIG. 5A ) a plurality of sets of first calibration parameters corresponding to intrinsic characteristics of the imaging device 12 using the calibration plate 502 set at a plurality of predetermined working distances 506a-506d and stores the first calibration parameters in a storage device (e.g., the mass storage device 210). For example, the processor 202 may determine and store calibration parameters for one or more of the focal length of the imaging device 12, the optical center of the imaging device 12, the distortion parameter(s), the pixel size(s) of the imaging device 12, and / or the parameter(s) representing pixel skew in the imaging device 12, for each of the desired working distances 506a-506d.

[0117] At block 1506, the optical extensometer 10 is delivered to the user. The delivered optical extensometer 10 includes an imaging device 12 that includes the same lens assembly that was used during calibration, and a computing device 32 that includes a storage device that includes the calibration parameters. In some examples, the delivered optical extensometer 10 also includes the load string that was used during calibration. In other examples, the load string may be different in the delivered system than during calibration. The example method 1500 then ends.

[0118] Upon delivery, the optical extensometer 10 is installed and a selected one of the predefined working distances 506a-506d is verified using the pre-calibrated intrinsic and extrinsic parameters. Figure 16 is a flow diagram depicting one example method 1600 that may be performed to verify the configuration of the installed optical extensometer system 10 of Figure 1 based on the stored intrinsic and extrinsic calibration parameters. Method 1600 is described below with reference to the example optical extensometer 10 of Figures 5B and 6A, but may also be performed using other optical extensometer systems.

[0119] At block 1602, the optical extensometer 10, including the load string 508, the imaging device 12, and the computing device 32, is installed in a desired orientation. For example, the imaging device 12 and the load string 508 may be positioned at one of the predetermined working distances 506a-506d that have been calibrated (e.g., by the manufacturer) prior to installation.

[0120] At block 1604, the optical extensometer 10 verifies (e.g., via the processor 202) the second calibration parameters corresponding to the orientation of the installed optical extensometer 10 using a second calibration plate (e.g., the verification plate 602 of FIG. 6A) and the first calibration parameters stored in the mass storage device 210. For example, the processor 202 may measure the current working distance and provide an indication that enables the operator or installer to adjust the working distance (e.g., via the adjustable imaging device mounts 1202, 1212 of FIGS. 12A and / or 12B) to precisely configure a selected working distance (e.g., within a threshold range) and / or make any other positional and / or orientation adjustments.

[0121] At block 1606, the operator may perform an optical deformation or distortion measurement process using the first calibration parameters and the second calibration parameters.

[0122] In block 1608, the operator or the processor 202 may determine whether a change has occurred in the placement of the installed optical extensometer 10. For example, the processor 202 may identify one or more factors that indicate that the working distance 604 (e.g., via monitoring a reference tab or similar device) or other aspect of the system has changed, or the operator may indicate via a user interface that such a change has occurred.

[0123] If no change in configuration is detected (block 1608), control returns to block 1606 to continue performing the measurement process. If a change in configuration has occurred (block 1608), control returns to block 1604 to again verify the configured working distance.

[0124] The method and system can be implemented in hardware, software, and / or a combination of hardware and software. The method and / or system can be implemented in a centralized manner in at least one computing system, or in a distributed manner where different elements are distributed across several interconnected computing systems. Any kind of computing system or other device adapted to perform the methods described herein is suitable. A typical combination of hardware and software can include a general-purpose computing system, together with a program or other code that, when loaded and executed, controls the computing system to perform the methods described herein. Another typical embodiment can include application specific integrated circuits, chips, or circuit-implemented logic. Some embodiments can include a non-transitory machine-readable (e.g., computer-readable) medium (e.g., flash drive, optical disk, magnetic storage disk, etc.), which stores one or more lines of code executable by a machine, thereby causing the machine to perform a process as described herein. As used herein, the term "non-transitory machine-readable medium" is defined to include all types of machine-readable storage media and to exclude propagating signals.

[0125] As used herein, the terms "circuitry" and "circuitry" refer to physical electronic components (i.e., hardware) and any software and / or firmware ("code") that may comprise, be executed by, and / or otherwise be associated with hardware. As used herein, for example, a particular processor and memory may include a first "circuitry" when executing a first one or more lines of code, and may include a second "circuitry" when executing a second one or more lines of code. As used herein, "and / or" refers to any one or more of the items in the list linked by "and / or". As an example, "x and / or y" refers to any element of the triplet {(x),(y),(x,y)}. In other words, "x and / or y" means "one or both of x and y". As another example, "x, y, and / or z" means any element of the seven-element set {(x),(y),(z),(x,y),(x,z),(y,z),(x,y,z)}. In other words, "x, y, and / or z" means "one or more of x, y, and z." As used herein, the term "exemplary" is meant to serve as a non-limiting example, instance, or illustration. As used herein, the term "for example" begins a list of one or more non-limiting examples, instances, or illustrations. As used herein, whenever circuitry includes the necessary hardware and code (if either is necessary) to perform a function, the circuitry is "operable" to perform that function, regardless of whether implementation of that function has been disabled or enabled (e.g., by a user-configurable setting, a factory trim, etc.).

[0126] Although the method and / or system have been described with reference to certain specific embodiments, those skilled in the art will recognize that various modifications and equivalents may be substituted without departing from the scope of the method and / or system. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the scope of the disclosure. For example, the systems, blocks and / or components of the disclosed examples may be combined, divided, rearranged, and / or otherwise modified. Thus, the method and / or system is not limited to the particular embodiments disclosed. Instead, the method and / or system includes all embodiments falling within the scope of the appended claims, both literally and under the doctrine of equivalents.

Claims

1. 1. An optical extensometer comprising: a load string configured to secure the test specimen; and an imaging device configured to capture an image of a surface of the test specimen when secured within the load string; a storage device configured to store a plurality of first calibration parameters corresponding to intrinsic characteristics of the imaging device; A control circuit unit, performing a verification process using the first calibration parameters to verify that a plurality of second calibration parameters corresponds to an arrangement of the test sample relative to the imaging device; performing an optical strain measurement process to measure a displacement of the test sample based on the first calibration parameters and the second calibration parameters; A control circuit unit configured as above; An optical extensometer comprising:

2. The optical extensometer of claim 1 , wherein the imaging device comprises a housing, an image sensor within the housing, and a lens configured to mount to the housing using a kinematic mount.

3. The optical extensometer of claim 2 , wherein the kinematic mount is configured to replicate a position and orientation of the lens corresponding to a calibration procedure associated with generating the first calibration parameter.

4. The kinematic mount comprises: a base having a plurality of seats; a lens mount including a plurality of bearings having complementary arrangements with respect to the plurality of seats; The optical extensometer of claim 2 , comprising:

5. The optical extensometer of claim 4 , wherein the base and the lens mount further comprise complementary keying features to prevent misalignment of the plurality of bearings with the plurality of seats.

6. 5. The optical extensometer of claim 4, wherein the base comprises a plurality of first magnets and the lens mount comprises a plurality of second magnets configured to interact with the first magnets to mate the plurality of bearings with the plurality of seats.

7. 10. The optical extensometer of claim 1, further comprising an adjustable imaging device mount configured to allow adjustment of a working distance between the imaging device and the load string.

8. 8. The optical extensometer of claim 7, wherein the imaging device comprises a housing, an optical sensor within the housing, and a lens mount having a fixed distance relative to the optical sensor, the adjustable imaging device mount being attached to the exterior of the housing.

9. 8. The optical extensometer of claim 7, wherein the imaging device includes a housing, an optical sensor within the housing, and a lens mount having a fixed distance relative to the optical sensor, and the adjustable imaging device mount is mounted inside the housing and configured to adjust the position of the optical sensor and the lens mount.

10. 8. The optical extensometer of claim 7, wherein the control circuitry is configured to perform the verification process and output a signal indicative of whether the working distance is within one or more predetermined working distance threshold ranges.

11. 2. The optical extensometer of claim 1, wherein the control circuitry is configured to calculate the second calibration parameter based on the first calibration parameter and using a predetermined validation specimen attached to the load string.

12. The optical extensometer of claim 1 , wherein the intrinsic characteristics of the imaging device include one or more of a focal length, an optical center, a distortion parameter, a pixel size, or a pixel skew parameter.

13. The optical extensometer of claim 1 , wherein the second calibration parameters include a relative position of the imaging device and the load string with respect to a reference position and a relative rotation of the imaging device and the load string with respect to a reference orientation.

14. The optical extensometer of claim 1 , wherein the control circuitry is configured to perform the verification process based on a predetermined calibration plate placed within the load string.

15. The optical extensometer of claim 1 , wherein the control circuitry is configured to perform the verification process based on a plurality of fiducial markers located on the load string.

16. 10. The optical extensometer of claim 1, further comprising an adjustable imaging device mount configured to allow adjustment of a focal length by adjusting a distance between a lens of the imaging device and an optical sensor, the adjustable imaging device mount comprising a plurality of discrete adjustment points.

17. 2. The optical extensometer of claim 1, wherein the storage device is configured to store an identifier corresponding to at least one component of the imaging device, the identifier being stored in association with the first calibration parameter, and the control circuitry is configured to perform the verification process using the first calibration parameter based on accessing the first calibration parameter in response to receiving the identifier at the control circuitry.

18. The optical extensometer of claim 17 , further comprising a communication device configured to receive the identifier from at least one of a removable lens of the imaging device or a remote control system.

19. 20. The optical extensometer of claim 18, wherein the communication device is configured to receive the identifier from the removable lens via at least one of a radio frequency identifier (RFID), a near field communication (NFC) tag, a secure data storage chip, or a programmable read only memory (PROM).

20. 2. The optical extensometer of claim 1, wherein the control circuitry is configured to select one of a plurality of sets of the second calibration parameters stored in the storage device corresponding to a plurality of predetermined working distances based on performing the validation process.

21. 1. An optical extensometer comprising: a load string configured to secure the test specimen; and an imaging device configured to capture an image of a surface of the test specimen when secured within the load string; a storage device configured to store a plurality of first calibration parameters corresponding to intrinsic characteristics of the imaging device; A control circuit unit, performing a calibration process using the first calibration parameters to calculate a plurality of second calibration parameters based on a position of the test sample relative to the imaging device; performing an optical extensometer process to measure a displacement of the test specimen based on the first calibration parameters and the second calibration parameters. A control circuit unit configured as above; An optical extensometer comprising:

22. 22. The optical extensometer of claim 21, wherein the imaging device comprises a housing, an image sensor within the housing, and a lens configured to be mounted to the housing using a kinematic mount, the kinematic mount configured to replicate a position and orientation of the lens corresponding to a calibration procedure associated with generating the first calibration parameter.

23. The kinematic mount comprises: a base having a plurality of seats and a plurality of first magnets; a lens mount comprising: a plurality of bearings having a complementary arrangement with respect to the plurality of seats; and a plurality of second magnets configured to interact with the first magnets to mate the plurality of bearings with corresponding ones of the plurality of seats; 23. The optical extensometer of claim 22, comprising:

24. 22. The optical extensometer of claim 21, wherein the control circuitry is configured to calculate the second calibration parameters based on a first calibration plate, the first calibration plate having a different set of visible features than a second calibration plate used to determine and store the first calibration parameters.

25. 22. The optical extensometer of claim 21, wherein the intrinsic characteristics of the imaging device include one or more of a focal length, an optical center, a distortion parameter, a pixel size, or a pixel skew parameter.

26. 22. The optical extensometer of claim 21, wherein the second calibration parameters include a relative position of the imaging device and the load string with respect to a reference position and a relative rotation of the imaging device and the load string with respect to a reference orientation.

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